Such high-pressure discharge lamps may be high-pressure sodium discharge lamps, and, more specifically, metal halide lamps having improved color rendition. The use of a ceramic discharge vessel for the lamps enables the use of the higher temperatures required for such vessels. The lamps have typical power ratings of between 50 W-250 W. The tubular ends of the discharge vessel are closed by cylindrical ceramic end plugs comprising a metallic current feedthrough passing through the axial hole therein.
Customarily, these current feedthrouhs are made of niobium tubes or pins (U.S. Ser. No. 07/954,815, filed Oct. 1, 1992, now U.S. Pat. No. 5,352,952, and EP-A 472 100). However, they are only partly suitable for lamps that are intended for a long useful life. This is due to the strong corrosion of the niobium material and, possibly, the ceramic material used for sealing the feedthrough into the plug when the lamp has a metal halide fill. An improvement is described in the European Patent Specification EP-PS 136 505 to which U.S. Pat. No. 4,545,799, Rhodes et al. corresponds. A niobium tube is tightly sealed into the plug by the shrinking process of the "green" ceramic during the final sintering without ceramic sealing material. This is readily possible because both materials have approximately the same thermal expansion coefficient (8.times.10.sup.-6 K.sup.-1).
Although metals such as niobium and tantalum have thermal expansion coefficients that match those of the ceramic, they are known for having poor corrosion resistance against aggressive fills and they have not yet been available for use as a current feedthrough for metal halide lamps.
Metals having a low thermal expansion coefficient (molybdenum, tungsten and rhenium) are the metals which have a high corrosion resistance against aggressive fills. Their use as a current feedthrough is, therefore, highly desirable. However, the problem of providing a gas-tight seal while using such feedthroughs has remained unsolved in the past.
It has already been attempted to use a molybdenum tube as a feedthrough (EP-PA 92 114 227.9; Art. 54(3) EPC to which U.S. Pat. No. 5,404,078, Bunk et al. corresponds). In order to avoid the use of ceramic sealing material which can be corroded by aggressive fill materials, the tube is gas-tightly sintered directly into the plug without any sealing material. This has to be done by a special manufacturing method. The best results are obtained by using a two-part feedthrough and/or a plug composed of two or more materials. Reference to the contents of that disclosure is expressly made, especially to the manufacturing method and to the composition of the plug material. In the said application the use of solid molybdenum pins is said to be disadvantageous because a pin cannot deform.
The use of a solid molybdenum pin as a feedthrough in connection with a ceramic vessel and plug, made from alumina, has also been discussed in the past. However, the gas-tightness between the plug and the pin is obtained by using a rather corrosion resistant sealing material (glass melt or ceramic melt) which is filled into the gap between the hole of the plug and the feedthrough (see for example U.S. Pat. No. 2,477,715, Claasens et al. Pin diameters of approximately, or not more than 600 .mu.m are used.
A detailed discussion of this technique is given in the U.S. Pat. No. 4,475,061 Van de Weiger et al. A molybdenum pin with a diameter of 0.7 mm is inserted into a plug having a hole of 0.8 mm diameter. Therefore, the gap between the pin and the plug wall is 0.05 mm. This gap, although in this application declared as being small, is quite big and facilitates the flowing of the sealing material--in this case, alkaline earth oxides--into the gap.
From DE-A 23 07 191, to which and U.S. Pat. No. 4,122,042 corresponds, a metal halide lamp is known which has a ceramic vessel with a plug made from a cermet consisting of alumina and molybdenum metal. A feedthrough of molybdenum is directly sintered into the plug. Obviously, this plug is electrically conductive because it is shielded from the discharge volume by a layer of insulating material which covers the surface of the plug facing the discharge volume.
This arrangement is disadvantageous because the metal halide fill can react with this material which also serves as a sealing material for the interface between the plug and the vessel end. As a consequence, a reliable long-time gas-tightness cannot be obtained and the maintenance of such a lamp is unsatisfactory.
Such lamps never came into use. The reason for this presumably is that these arrangements were unable to provide for protection against the inevitable corrosion of the sealing material.